1. Field of the Invention
The present invention relates to a superplastic silicon carbide sintered body and a method for its preparation. More particularly, the present invention relates to a sintered body whereby silicon carbide excellent in corrosion resistance and abrasion resistance even at a high temperature, can directly be processed by plastic processing into various shapes, such as chemical plants, pumps for transporting liquids and engine parts, and a method for preparing such a sintered body.
2. Discussion of Background
Ceramics have excellent properties such as high strength and high hardness at both low and high temperatures, as compared with organic polymer or metallic materials, and their application has been expanded to parts such as structural materials to be used at low or high temperatures. Especially, silicon carbide ceramics have been developed for practical application as corrosion and abrasion resistant mechanical parts to be used under severe conditions. However, ceramics materials including silicon carbide ceramics have a problem that it is difficult to process them. Namely, metal materials can readily be molded by rolling or casting and can readily be machine-processed by e.g. cutting or machining, and thus, they have a feature that the processing cost is low. Whereas, ceramics can hardly be processed by bending or cutting, and they are obliged to be processed by grinding or polishing. Therefore, a tool made of diamond is required, whereby the processing cost tends to be extremely high, and such a cost may sometimes amount to more than one half of the price of the parts.
To reduce such processing cost of ceramics, it is necessary to sinter and form parts in a shape as close as possible to their final shape so that the subsequent processing can be minimized. For this purpose, it is desired to prepare ceramics to have super-fine grains, so that their superplasticity can be utilized. Here, the superplasticity usually means a nature such that superfine crystal grains of metal readily undergo plastic deformation under an external stress at a high temperature. By utilizing this superplasticity, it is possible to readily produce various products ranging from materials having simple shapes to parts having complicated shapes. Accordingly, if such superplasticity can be utilized also with respect to ceramics, parts as final products can be prepared by the same means as in the case of metals, whereby the processing cost can substantially be reduced. From such a viewpoint, a study has been made for near-net shaping to obtain a desired shape by utilizing the superplasticity also in the case of ceramics.
Heretofore, it has been known that super-fine grains of zirconia, mullite and apatite among oxide ceramics exhibit superplasticity, and development has been made for practically useful parts utilizing the superplasticity.
Further, the present inventors have previously found that when silicon nitride of a fine grain size is sintered at low temperature, it can be sintered while the grains are maintained to be fine and uniform, and on the basis of this discovery, they have developed a sintering method for superplastic silicon nitride. However, in the case of silicon carbide, a high density sintered body cannot be obtained unless it is sintered at a temperature higher by 200.degree. C. than silicon nitride, and if it is sintered at such a high temperature, grain growth of silicon carbide will take place, and the average particle size will be at lest 2 .mu.m, thus leading to a difficulty that such a product will not be deformed at a temperature lower than the sintering temperature. The only report is that silicon carbide shows a deformation of about 40% at 1,900.degree. C. (C. Carry and A. Mocollin. Mater. Res. Soc. Vol. 18. p. 391 (1984)). However, this temperature is substantially the same as the sintering temperature of silicon carbide. The deformation rate is usually highly sensitive to the temperature, and most materials tend to readily deform when the temperature becomes close to their sintering temperatures. Therefore, this report is not related to plastic deformation at a temperature lower than the sintering temperature. Superplasticity is significant technically as well as academically when deformation is shown at a temperature substantially lower than the sintering temperature. From this viewpoint, a superplastic silicon carbide sintered body has not yet been developed by the study so far made. Silicon carbide is expected to be useful as a material which has high hardness and which exhibits excellent corrosion resistance and abrasion resistance even under severe conditions at low or high temperatures. Therefore, it has been very much desired to realize a new technology whereby such silicon carbide can be subjected to plastic deformation and can be made into shapes close to the final shapes of various parts having complicated shapes, so that post processing such as grinding can be minimized to produce the final products at low cost.